Using QSqlQuery from multiple threads

问题

I have a lot of C++11 threads running which all need database access at some time. In main I do initalize the database connection and open the database. Qt documentation says that queries are not threadsafe so I use a global mutex until a QSqlQuery exists inside a thread.

This works but is that guaranteed to work or do I run into problems at some time?

回答1:

A look at the Documentation tells us, that

A connection can only be used from within the thread that created it. Moving connections between threads or creating queries from a different thread is not supported.

So you do indeed need one connection per thread. I solved this by generating dynamic names based on the thread:

auto name = "my_db_" + QString::number((quint64)QThread::currentThread(), 16);
if(QSqlDatabase::contains(name))
    return QSqlDatabase::database(name);
else {
    auto db = QSqlDatabase::addDatabase( "QSQLITE", name);
    // open the database, setup tables, etc.
    return db;
}

In case you use threads not managed by Qt make use of QThreadStorage to generate names per thread:

// must be static, to be the same for all threads
static QThreadStorage<QString> storage;

QString name;
if(storage.hasLocalData())
    name = storage.localData();
else {
    //simple way to get a random name
    name = "my_db_" + QUuid::createUuid().toString();
    storage.setLocalData(name);
}

Important: Sqlite may or may not be able to handle multithreading. See https://sqlite.org/threadsafe.html. As far as I know, the sqlite embedded into Qt is threadsafe, as thats the default, and I could not find any flags that disable it in the sourcecode. But If you are using a different sqlite version, make shure it does actually support threads.

回答2:

You can write class with SQL functions and use signals-slots to do the queries and get result from database.

It's thread-safe also no need to use mutex.

回答3:

You choose not well approach. Should use shared QSqlDatabase object instead QSqlQuery. Please check next example of multithreading database access. If that will not clear for you please let me know. Will explain more.

来源：https://stackoverflow.com/questions/47457478/using-qsqlquery-from-multiple-threads

标签

multithreading

c++11

qtsql